Polyester films are excellent in heat resistance, chemical resistance, electric insulation and the like, and hence are used in wide fields of packaging film, film for magnetic tape, optical film, film for electronic component and the like.
Recently, outer packaging materials for laminate-type lithium ion batteries, press-through packs and the like have been obtained by cold molding of laminates constituted with resin films and metal foils.
In general, the laminates for performing the cold molding adopt constitutions such as nylon film (Ny)/Al foil/undrawn polypropylene film (CPP), and polyethylene terephthalate film (PET)/Ny/Al foil/CPP; laminates including Al foil have nylon film as laminated therein in order to impart ductility for enabling cold molding.
However, the lamination of a polyamide film directly causes cost increase in production of laminates, moreover unfortunately degrades physical properties due to thermal deterioration under high temperatures and high humidities because polyamide films are poorer in thermal resistance as compared with polyester films, and also disadvantageously causes dimensional changes due to moisture absorption because polyamide films are hygroscopic, thus unfortunately, for example, causing concern about curl in resulting packaging bags.
On the other hand, a polyester film is harder and brittler than a nylon film, and has a large anisotropy because of being generally produced by a tenter-type successive drawing method; thus, it has been difficult to impart ductility to a metal foil laminated on such a polyester film. However, polyester films capable of constituting laminates excellent in cold moldability have been proposed; for example, Patent Literature 1 and Patent Literature 2 each disclose a polyester film for packaging lithium batteries, the polyester film being specified in such a way that the stresses at elongation in the lengthwise direction and widthwise direction of the film fall within specified ranges. Recently, as laminates used for outer packaging materials for laminate-type lithium ion batteries, press-through packs and the like, laminates having a constitution such as PET/Al/CPP using only a polyester film for the outer layer thereof without using any nylon film have been adopted.
In a laminate including a resin film and a metal foil, it is important to impart ductility to the metal foil by the resin film when cold molding is performed, and for that purpose, the resin film is required to extend uniformly in all directions. When a resin film has variations in the physical properties in the four directions, namely, MD, a 45° direction, TD, and a 135° direction, it is difficult to extend the resin film uniformly in an the directions during cold molding. Specifically, when the resin film has an easily extending direction and a hardly extending direction, the metal foil is broken during cold molding, or alternatively delamination or generation of pin holes occurs in the resin film. When such a problem occurs, the molded article cannot perform the functions as a package or the like, and such a problem is liable to lead to the damage of the packaged object (the content). Therefore, the resin film is required to be reduced as much as possible in the variations of the physical properties in the respective directions.
As the factors affecting the moldability during the cold molding, the flexibility of the resin film is quoted. When the resin film is low in flexibility, a strong load is applied to the resin film at elongation in cold molding, and pin holes and delamination possibly occur. Conversely, when the resin film is too high in flexibility, the effect as a substrate to protect the laminate including the metal foil is reduced, and the obtained laminate is degraded in physical properties. Therefore, it is important that the resin film has a flexibility not too high and not too low.
Additionally, the thickness of the resin film is to be mentioned as one of the other physical properties affecting the moldability during the cold molding. When a laminate including a polyester film having thickness variation as laminated is cold molded, the relatively thinner portion of the polyester film is highly liable to be broken to form pinholes or to cause delamination. Therefore, it is important that the polyester film used for cold molding is controlled to be uniform in thickness all over the film.
Recently, resin films used for outer packaging materials for lithium ion batteries and laminates of such films have been required to be more reduced in resin film thickness, with the request for the achievement of further higher output powers of batteries, the miniaturization of batteries, and the cost reduction for batteries. In general, resin films ensure the thickness uniformity more easily with the increase of the thickness; however, with the decrease of the thickness (in particular, when the thickness is 25 μm or less), the uniformity of the thickness is more degraded, and accordingly the effect on the moldability is more remarkable.
As described above, the development of a polyester film has been earnestly desired as the resin film being comparatively small in the variations of the physical properties in the above-described four directions, having a flexibility falling in an appropriate range, moreover being excellent in the thickness uniformity, and at the same time, having a satisfactory cold moldability although being thinner however, as affairs stand, such a film has never been developed.